Patent GB-476311-A (GB)
Title: Device for the conversion of an image formed by lightrays into electrical impulses
IPC: H01J31/28
Filed: 1936-06-04
Abstract: 476,311. Cathode-ray tubes. KOEMPFNER, R. June 4, 1936, Nos. 15620, 20756, and 22732. [Classes 39 (i) and 40 (iii)] A cathode-ray tube is provided with a photo-sensitive grid which is disposed across the path of an electron stream and is of such a character that when an optical or like image is projected on it, transverse local electrostatic fields are established the strength of which at each point of the grid depends upon the intensity of the image at the point in question, means being provided for influencing the paths of the electrons which have passed through the grid so that when the latter is not illuminated these electrons are converged and come substantially to a focus at a fixed point. In a modification, the electrons passing through the grid are deflected so as to fail to pass through an aperture in a shield electrode when the grid is not illuminated, but the electrons passing through a fully illuminated part of the grid also pass through the shield aperture to fall upon a fluorescent screen. An optical image is projected on the photosensitive grid 4 of the tube 1 by a lens system 5, comprising cylindrical lenses which compress the image produced and so compensate for error introduced by the inclination of the grid, and the grid is scanned by a beam of cathode rays produced by the gun 2. So long as the grid is unilluminated, the electrons which pass therethrough are deflected by secondary deflecting means 11, which may be electrostatic or electromagnetic, to an electrode 3 which constitutes an output electrode for television signals ; if, however, any part of the grid is illuminated, the local field set up disperses the electrons passing through the grid at that point to a degree which depends on the illumination. The dispersed electrons are collected by a metallic coating 6 which may also serve as the output electrode. In a modification, the electrons passing through the unilluminated grid are concentrated on to an aperture in a shield electrode arranged at the end of the tube and an output electrode may be arranged behind the aperture ; alternatively this output electrode may be replaced by a fluorescent screen on which a negative picture for monitoring purposes may be obtained, the signals then being taken off the shield electrode or a thin metallic mesh located in front of the screen. The signals may be amplified by making the electrode behind the apertured-shield electrode the first electrode of an electron multiplier. Instead of using a thin electron beam for scanning the grid, the latter may be irradiated over its whole surface by a divergent stream of electrons, those which pass through the grid being deflected so that when the latter is unilluminated they pass through an aperture in a shield electrode, are refocused and accelerated respectively by a tubular and an apertured electrode and finally fall upon a fluorescent screen, the device then serving as an image converter. To convert the negative image so produced into a positive image, the fluorescent picture may be used as a source to illuminate the grid of a second and similar tube, or the fluorescent screen may be replaced by the photo-sensitive grid of a second tube, the grid yielding secondary electrons on impact and thus producing local fields which are used to influence the electron stream from the second cathode-ray gun ; or alternatively the metallic cores of the photo-sensitive elements of the grid may be given an initial negative bias, the electrons in this case being deflected after passing through the grid so as to pass through the aperture in the shield electrode when the grid is fully illuminated. The grid may be discharged by leakage or by means of an auxiliary beam of electrons from a gun located in a side branch of the tube near the end remote from the main gun, and this auxiliary beam may either irradiate instantaneously the whole surface of the grid at specific intervals, or may be formed into a thin pencil which is caused to scan the grid. The latter may be composed of woven threads, metallic or conductive threads alternating with insulating threads formed of quartz or of insulating sleeves of aluminium oxide on aluminium wire cores, the insulating threads being provided with globules of oxidized-silver coated with caesium. The threads may be parallel and co-planar or arranged in two parallel planes, or they may be interwoven at any angle. Alternatively, the grid may consist of tungsten threads covered with insulating-material, alternating with bare tungsten threads, and the former may be provided with globules of oxidized silver formed by using glass or quartz filaments sprayed with a conducting, non-elastic coating which on stretching breaks up into separate elements. Alternatively, the silver globules may be obtained by photo-chemical means. The bare metallic threads of the grid may be maintained at a potential approximately equal to that of the last anode of the electron gun and a suitable bias may be applied to the insulated threads. In a modified construction of grid an insulating, perforated membrane may be used, having photo-sensitive globules on one side and an electrically conductive mesh on the other side, arranged so as not to obstruct the openings in the membrane.